The general form of the well-known Fierz identities (see, for
instance, Ref. [1]}, [2]}) allows us to
disregard contractions that are not in a standard order (though this
is arbitrarily chosen). Any operator obtained by reordering the
pattern of contractions will be a linear combination of the previous
operators. This argument also applies to more than two bilinears,
since a general permutation of fields can be expressed as a number of
transpositions, for which the usual formula holds.
